[106.10] Far-infrared and sub-mm observations of the Crab nebula

We present FIR and sub-mm images of the Crab nebula, taken
with the ISOPHOT instrument on board ISO at 60, 100, and 170
micron, and with the JCMT at 850 micron. The 60 and 100
micron images show a clear excess of emission above the
extrapolation of the synchrotron spectrum from lower
frequencies, as previously seen using IRAS. The ISOPHOT
images reveal that about half this excess arises from two
peaks not seen on synchrotron images, separated by ca. 80
arcsec. The 170 micron image does not show any excess
emission. Spectroscopic measurements show that this FIR
excess is not due to line emission. We interpret the FIR
excess as emission from a small amount of warm dust (in the
ranges 0.01-0.07 and 0.003-0.02 Msolar for silicate and
graphite, respectively), superimposed on a synchrotron
spectrum which gradually steepens towards shorter
wavelengths throughout the FIR and MIR. The dust geometry is
consistent with a torus of diameter ca. 0.8pc created by the
supernova progenitor prior to its explosion, superimposed
upon a broadly distributed component which may be supernova
condensates in the filaments. Our upper limit on the total
mass of graphite is consistent with the inference from
gas-phase Carbon abundances that there has been no
significant enrichment of the filaments in Carbon
nucleosynthesis products from the progenitor. Comparison of
the 850 micron image with a 20-cm VLA image shows that the
only variation in spectral index of the synchrotron emission
across the remnant is a small change near the centre. Given
the good agreement between the integrated flux density at
850 micron and the extrapolated synchrotron spectrum,
together with the different epochs of the 850 micron and
20cm images, we do not see the need for the second radio
synchrotron component which has previously been proposed.